Electrical characterization of deep defect states in Galliumnitride co-implanted with magnesium and sulfur ions

GaN layers grown by metal organic vapor phase epitaxy on sapphire were implanted with 90 keV magnesium ions to achieve p-type conductivity. However, besides the magnesium acceptors the implantation process also induces additional defects with compensating impact preventing the p-type conductivity wanted. Our approach to reduce this compensation effect bases on defect passivation by sulfur realized by a co-implantation with 150 keV sulfur ions in different implantation sequences. The corresponding implantation induced defect states were characterized by various electrical and photoelectrical spectroscopy techniques, e.g. deep level transient spectroscopy (DLTS) and admittance spectroscopy. Independent of the implanted ion species and of the implantation sequences we found four implantation induced electron traps with thermal activation energies between 140 meV and 1.1 eV which are typical for all single and co-implanted layers. The depth resolved concentration profile of these intrinsic defects determined by DLTS at different bias voltages corresponds either to the profile of the introduced ions or to those of the generated vacancies as predicted by simulations. Besides this normal implantation induced defect generation we found indications for passivation effects due to sulfur in our optical admittance spectra where contributions from deep states as well as from the magnesium acceptor can be drastically suppressed. Remarkably, these effects significantly depend on the implantation sequence, e.g. whether magnesium or sulfur ions are implanted finally.